Choosing between skylights for flat roofs isn’t like choosing a standard pitched roof window. A flat roof needs a different kind of skylight: instead of tilting into a slope, it sits on a raised upstand and is built to shed water off a much shallower pitch, typically 0° to 15°. That single difference is why so many buying guides for skylights don’t actually apply to a flat roof, and why choosing the wrong type is a common mistake.
This guide does two things most others don’t combine. First, it walks through every type of skylight suited to a flat roof, so you can work out which one fits your room and your roof. Second, it shows real prices, U-values, warranties and lead times for actual products you can order today, so you don’t need to leave the page and request a quote just to find out what something costs.
What counts as a skylight for a flat roof
A skylight for a flat roof is designed to sit on an upstand, a raised timber or PVC frame that lifts the unit above the roof surface so water drains away rather than pooling against the glass. This is different from a pitched roof window, which is fitted flush within the roof slope and relies on the pitch itself to shed water.
The terms skylight, rooflight and roof window get used almost interchangeably in the UK, though roof window technically refers to units designed to be installed in the same plane as the surrounding roof, as set out in BS EN 14351-1. We cover this distinction in more depth in What’s the Difference Between Skylights, Rooflights and Roof Windows? For a flat roof application, the naming matters less than getting the right pitch range, upstand height, and position for your specific roof; our guide on the best place to position a skylight covers orientation and room-by-room placement.
Types of skylights for flat roofs
Flat roof skylights
A flat roof skylight uses a single pane of toughened or laminated glass set flush within its frame, fitted onto an upstand. It’s a common choice for flat roof extensions, and for loft conversions specifically where the loft has a flat or near-flat roof structure, since a standard pitched loft doesn’t take this type of unit. It gives an unbroken view of the sky, with no visible framing lines from below on frameless designs. TGS’s own Flat Rooflight, Framed Rooflight TGS, and Framed Flat Skylights all sit in this category, alongside the Korniche Flat Lite and Coxdome Flat Glass Rooflight. Whether a framed or frameless design suits your project better is covered in our framed vs frameless rooflights guide.
The main trade-off is cost scaling with size. A small fixed unit is inexpensive, but larger sizes and triple glazing add up quickly, and very large spans may need a modular system instead of a single pane. It’s also worth understanding what separates a genuinely well-built unit from a cheaper import; our director’s take on the hidden cost of cheap rooflights covers where corners tend to get cut.
Fixed skylights
Fixed skylights don’t open. Their only job is to let in light, which means fewer moving parts, a simpler seal, and a lower price than an equivalent opening unit. Browse our fixed rooflights range for standard sizes and pricing.
The trade-off is exactly what you’d expect: no ventilation. That’s a genuine limitation in kitchens, bathrooms, or any room prone to condensation, where a sealed unit can trap moisture against the glass over time. Our full breakdown of opening vs non-opening skylights weighs this trade-off in more detail.
Opening skylights
Opening skylights add a manual crank handle or an electric motor to the same basic flat glass or dome format, letting stale air out and fresh air in. See our opening rooflights range for manual and electric options.
They cost more than fixed units because of the added hardware, and electric versions carry an ongoing risk of motor or remote-control faults that a fixed skylight simply doesn’t have. Read our comparison of electric vs manual opening skylights if you’re deciding between the two.
Roof lanterns
A roof lantern projects above the roofline with a pitched, multi-paned glass structure, rather than sitting flush like a flat rooflight. That raised profile lets light in from more angles throughout the day and gives a room a more architectural feature than a flat pane provides. Our roof lanterns range includes the Korniche Roof Lantern, Wendland Roof Lantern, and Brett Martin Roof Lantern.
The trade-off is installation complexity and heat loss. Because a lantern is heavier and larger than a flat unit, it usually needs more structural reinforcement, and its raised profile means more exposed surface area for heat to escape through.
Walk-on rooflights
A walk-on rooflight uses reinforced glass built to take foot traffic, making it suitable for roof terraces, balconies, and basement light wells that double as a floor surface. See our Walk-On Rooflights product page for sizes and pricing.
This durability adds real cost. Walk-on units run considerably more expensive than a standard flat rooflight of the same size, and the glass needs a slip-resistant coating to remain safe in wet conditions, which is worth confirming before ordering.
Pyramid skylights
A pyramid skylight uses four sloped glass panels meeting at a central point, which distributes daylight evenly through the day as the sun moves, unlike a flat pane that can create a single bright patch at certain times of day. Browse our pyramid skylights range for available sizes.
The sloped design sheds water effectively, but the additional panels and structural framing typically put pyramid units toward the higher end of the price range for a given base size.
Dome and polycarbonate rooflights
Dome rooflights use a curved polycarbonate cover rather than flat glass, most commonly seen in single, double, triple, or quad-skin configurations. They’re a lower-cost alternative to flat glass and are frequently used on garages, extensions, and commercial roofs where budget matters more than the clearest possible view of the sky. Our domed rooflights range and dedicated Coxdome range cover the Mardome Trade Polycarbonate Dome, Mardome Reflex Dome Only, Coxdome Galaxy Dome Only, and Coxdome Polycarbonate Dome.
Polycarbonate doesn’t match glass for optical clarity or long-term appearance, and can discolour slightly over many years of UV exposure, though it offers strong impact resistance in exchange.
Circular and modular rooflights
Circular rooflights are exactly what the name suggests: a round glass unit, typically available in a small number of fixed diameters rather than custom sizing. See our Circular Rooflights product page for the available sizes. Modular rooflights link multiple glazed panels together to cover very large spans, commonly used on commercial buildings or ambitious residential extensions where a single pane isn’t practical; our Modular Rooflights range covers this.
Both come with longer lead times than standard rectangular units, since they’re lower-volume products manufactured to fewer standard configurations.
Glazing and materials: glass vs polycarbonate, double vs triple
The glazing type you choose affects durability, insulation, and price more than almost any other decision. Toughened glass is the standard choice for its impact resistance in poor weather. Laminated glass adds a safety interlayer that holds the pane together if it’s ever broken, which is why several TGS products, including the Framed Flat Skylights range, use a laminated inner pane as standard rather than as an upgrade.
Double glazing pairs two panes with an argon-filled cavity, while triple glazing adds a third pane and a second cavity for better thermal performance. The difference shows up directly in U-value: TGS’s own Flat Rooflight moves from a U-value of 1.1 in double glazing to 0.9 in triple, and the same pattern holds across most flat glass ranges. Triple glazing costs more, but it reduces heat loss in winter and heat gain in summer, which matters more in south-facing rooms or larger glazed areas.
Compare real products: prices, U-values, warranties and lead times
The table below covers our most popular flat roof skylights, rooflights, and lanterns available from Toughened Glass Systems.
| Product | Price range (ex VAT) | Typical U-value | Warranty | Lead time |
|---|---|---|---|---|
| Framed Rooflight TGS | £350.00 – £2,733.20 | 1.2 (double) / 0.7 (triple) | 10 years | 2 to 5 days (first 10 sizes) |
| Framed Flat Skylights | £420.28 – £3,558.06 | 1.1 | 10 years (20-year design life) | 2 to 3 days (standard sizes) |
| Triple-Glazed Self-Cleaning Rooflight | £163.00 – £619.00 | 0.9 | 20 years | 1 to 3 days |
| Flat Rooflight | £259.00 – £2,800.00 | 1.1 (double) / 0.9 (triple) | Manufacturer’s warranty | 2 to 5 days (popular sizes); 7 to 12 days (other sizes) |
| Korniche Flat Lite | £109.50 – £875.10 | Not stated numerically; PAS 24 rated | 10 years | 2 to 3 weeks |
| Coxdome Flat Glass Rooflight | £577.30 – £1,484.65 | 1.0 | 10 years | 3 to 5 days |
| Korniche Roof Lantern | £694.56 – £4,247.01 | 1.0 to 1.2 | 10 years | 2 to 3 weeks |
| Wendland Roof Lantern | £449.45 – £2,009.28 | 1.0 to 1.2 | 10 years | 5 to 10 days |
| Brett Martin Roof Lantern | £915.28 – £1,946.94 | Meets Part L minimum | 10 years | 3 to 5 days (clear 4-pane); 10 to 15 days (other specs) |
| Mardome Trade Polycarbonate Dome | £115.00 – £1,853.17 | Varies by skin count | 10 years | 2 to 3 days (standard sizes) |
| Mardome Reflex Dome Only | £123.63 – £1,145.28 | Varies by skin count | 10 years | 2 to 3 days |
| Coxdome Galaxy Dome Only | £131.79 – £4,194.75 | 1.7 (triple skin) | 10 years | 2 to 3 days (2 to 3 weeks for quad skin) |
| Coxdome Polycarbonate Dome | £241.69 – £1,433.48 | 1.4 to 2.7 depending on skin count | 10 years | 2 to 3 days |
| Walk-On Rooflights | £869.00 – £3,575.00 | 1.1 | Manufacturer’s warranty | 15 to 23 days |
| Circular Rooflights | £977.13 – £3,230.39 | 1.16 | 10 years | 15 to 30 days |
| Modular Rooflights | £2,348.98 – £15,589.91 | 1.31 to 1.65 | 10 years (20-year design life) | 4 to 6 weeks |
A few patterns worth noting from the table. Lead time correlates more closely with how standardised a product is than with its price: the Korniche Flat Lite costs less than several dome options but takes 2 to 3 weeks because it’s made to order, while standard-size dome rooflights ship in 2 to 3 days. Warranty length is consistent at 10 years across almost the entire range, with the Triple-Glazed Self-Cleaning Rooflight standing out at 20 years.
If you already know which type suits your project and just want to browse and buy, the Skylights for Flat Roofs category page is built for exactly that. This section is for comparing your options; that page is for ordering once you’ve decided.
Energy efficiency and U-values
U-value measures how much heat passes through a skylight, expressed in W/m²K. The lower the number, the better the insulation: less heat escapes in winter, and less heat builds up in summer. Building Regulations set minimum energy efficiency standards for replacement windows and rooflights under Approved Document L, which is the reference point installers and building control officers work from.
Looking at the comparison table, triple glazing consistently brings U-values down: TGS’s own Triple-Glazed Self-Cleaning Rooflight achieves 0.9, while double-glazed dome rooflights sit closer to 2.7 depending on skin count. A lower U-value costs more upfront but reduces the ongoing heating and cooling load on the room beneath it, which matters most in larger glazed areas or rooms with significant south-facing exposure. For a fuller breakdown of U-values and our top picks by performance, see How to Choose an Energy-Efficient Skylight.
Ventilation and airflow
Ventilated skylights help regulate indoor air by letting warm, stale air escape while drawing in fresh air, which matters most in kitchens, bathrooms, and any room prone to condensation. Electric versions add convenience through remote operation and often include rain sensors that close the unit automatically, though that convenience comes with the maintenance considerations covered in our electric vs manual opening skylights guide.
Installation and building regulations
Most flat roof skylights fall under permitted development in England and Wales, meaning planning permission usually isn’t required, provided the installation doesn’t project more than 150mm above the existing roof plane and doesn’t exceed the height of the roof itself. The full conditions are set out on the Planning Portal’s roof planning permission page. Listed buildings, conservation areas, and properties with an Article 4 direction removing permitted development rights are the main exceptions, and it’s worth checking with your local planning authority before ordering if any of those apply.
Even where planning permission isn’t needed, Building Regulations sign-off still applies, covering structural loading, weatherproofing, and the energy efficiency standards referenced above. The Planning Portal’s building regulations guidance for rooflights covers what building control will check.
Most standard-size units can be fitted by a competent builder rather than requiring a specialist installer, though roof lanterns and larger modular systems typically need more structural assessment given their added weight. We cover the full detail of measuring, planning, and installing in our dedicated guides: Can You Install a Rooflight on a Flat Roof? and How Much Does it Cost to Install a Flat Rooflight?
Maintenance and lifespan
Routine maintenance is straightforward: clean the glass periodically, inspect seals and flashing at least once a year, and check for early signs of condensation, which can indicate a ventilation issue rather than a fault with the unit itself. Warranty length is a reasonable proxy for expected lifespan. Most products in the comparison table above carry a 10-year warranty, with the Triple-Glazed Self-Cleaning Rooflight and the Framed Flat Skylights range extending further, the latter with a stated 20-year design life alongside its 10-year guarantee. Our guide on how long rooflights last covers the factors that affect this beyond the warranty figure itself.
How much do flat roof skylights cost
Based on the comparison table above, product prices range from around £109 for a small fixed Korniche Flat Lite up to over £15,000 for a large modular system spanning a commercial roof. Most single-unit domestic projects fall between £150 and £3,500 depending on size, glazing, and whether the unit opens.
That’s the product cost alone. Installation adds to the total, and the amount varies significantly by type: a standard flat rooflight fitted into an existing upstand is a more modest job than a roof lantern requiring new structural support. Our full cost breakdown guide covers installation pricing in detail rather than repeating it here.
Why choose Toughened Glass Systems
Toughened Glass Systems supplies named brands including Korniche, Mardome, Coxdome, Wendland, and Brett Martin, alongside our own manufactured flat rooflight range. We offer a Best Price Guarantee matching like-for-like prices on equivalent products, alongside UK-wide delivery.
If you’re weighing us up against a specific brand, our Velux vs Fakro comparison and best Velux alternatives guide cover how our range stacks up.
Ready to browse and buy? Visit our Skylights for Flat Roofs category page for the full range, or contact our team for advice on your specific project.
Related guides
- What Is a Skylight? The Complete Guide to Skylights, Rooflights and Roof Windows
- Can You Install a Rooflight on a Flat Roof?
- How Much Does it Cost to Install a Flat Rooflight?
- Electric vs Manual Opening Skylights: A Detailed Guide
- Advantages and Disadvantages of Flat Roofs
- Pros and Cons of Flat Roofing for Commercial Properties
- Flat Roof Extensions: A Complete UK Guide for 2025
- Flat Roof vs Pitched Roof Skylight Extensions
- Flat Roofs: Complete Guide
Frequently asked questions
Do flat roof skylights need planning permission?
Usually not. Most installations fall under permitted development, provided the unit doesn’t project more than 150mm above the roof plane and isn’t higher than the roof itself. Listed buildings, conservation areas, and properties under an Article 4 direction are the main exceptions. Check the Planning Portal’s guidance for the full conditions.
What’s the difference between a skylight, a rooflight, and a roof window?
In everyday UK usage, the three terms are largely interchangeable. Technically, a roof window is designed to sit in the same plane as the surrounding roof, as defined in BS EN 14351-1, while rooflight and skylight are broader terms covering flat, domed, and lantern designs alike.
What is the minimum roof pitch for a flat roof skylight?
Flat roof skylights are typically designed for pitches between 0° and 15°. Below that range, water drainage relies entirely on the upstand height and the unit’s own drainage design, so check the specific product’s minimum pitch requirement before ordering.
How much does a flat roof skylight cost, installed?
Product cost alone ranges from around £109 to several thousand pounds depending on size and type, as shown in the comparison table above. Installation cost depends on the complexity of the job; see our dedicated cost guide for a full breakdown.
Can I fit a flat roof skylight myself?
Standard-size units can often be fitted by a competent builder without specialist skylight installation experience, provided the upstand is already correctly built and sized. Roof lanterns, walk-on rooflights, and modular systems are more likely to need structural input given their added weight and size. See our guide on installing a rooflight on a flat roof for the full process.
What U-value should I look for?
Lower is better. Triple-glazed products in the 0.7 to 0.9 range offer the best thermal performance in this comparison, while double-glazed domes typically sit between 1.4 and 2.7 depending on skin count. Building Regulations under Approved Document L set the minimum standard your installation needs to meet.
Do flat roof skylights leak, and how is that prevented?
A correctly installed skylight on a properly sized upstand, with correctly fitted flashing, shouldn’t leak. Most leaks trace back to an undersized upstand, poor flashing, or water pooling due to insufficient roof fall rather than a fault with the skylight itself.
Glass or polycarbonate: which is better?
Glass offers better optical clarity and doesn’t discolour over time, and is the better choice where appearance matters. Polycarbonate domes cost less and offer strong impact resistance, making them a practical option for garages, commercial roofs, or budget-driven projects where clarity is a lower priority.
How long do flat roof skylights last?
Most products in this range carry a 10-year warranty, and several, including the Triple-Glazed Self-Cleaning Rooflight and Framed Flat Skylights, extend further, up to 20 years. Actual lifespan depends heavily on installation quality and ongoing maintenance.
What size skylight do I need for my room?
This depends on the room’s floor area, its existing window provision, and how much additional light you want. As a general guide, aim for roughly 15 to 20% of floor area in glazing for a well-lit result, though this varies by room orientation and use. Where you position it matters as much as size; see our guide on the best place to position skylights for room-by-room guidance.
Can I install a walk-on rooflight on a roof terrace?
Yes. Walk-on rooflights are specifically designed and reinforced for foot traffic, making them suitable for roof terraces, balconies, and basement light wells that double as a walking surface. They cost more than standard flat rooflights due to the reinforced glass required.
Electric or manual opening: which is better?
Manual units cost less and have fewer parts to maintain. Electric units add convenience, remote operation, and often a rain sensor that closes the skylight automatically, at the cost of occasional motor or control servicing. See our full comparison for more detail.
How do I stop condensation on a flat roof skylight?
Condensation usually points to a ventilation issue rather than a fault with the glass. Opening skylights help by allowing moist air to escape, and some units include built-in ventilation channels or coatings designed to reduce moisture buildup. Read our guide to stopping condensation on rooflights for more detail.
Do I need a structural engineer to install a roof lantern?
Often, yes, particularly for larger spans. A roof lantern’s weight and size mean the surrounding roof structure usually needs assessment to confirm it can take the additional load, which is a different consideration from a standard flat rooflight fitted into an existing opening.